In conventional open-cycle operation of such a system, with each vehicle circulating between a loading station and an unloading station assigned to it at the beginning of the shift, impaired efficiency may result from failure of the transport fleet to keep pace with production of a given excavator or from assignment of excessive numbers of vehicles to an associated loading station. This mode of operation therefore may prevent attainment of a given production quota. It is, furthermore, desirable that the traffic stream entering the loading and unloading stations be as uniform as possible to prevent local congestions.
In practice, however, it is very difficult in open-pit mines to determine and provide for an exact number of transporting vehicles required to meet the shift quota. Consideration must also be given to the need for maintaining a planned ratio of ore output to the inevitable production of overburden, as well as to possible changes in the number of available vehicles. Another important requirement is proper distribution of the ore according to the quality or grade thereof, bearing in mind the various processing facilities of the plant and the specifications of different consumers. Thus, for example, with widely scattered iron-ore deposits differing in their phosphorus content, predetermined fractions of the outputs of the several sites may have to be blended in order to obtain a homogenized commercial product of specific composition.